172 THE ARTISTIC ANATOMY OF ANIMALS 



extensor of the forearm (caput parvum) to the external head ; 

 and of medium extensor of the forearm (caput medium) to the 

 internal.* 



It is more especially the long portion and the external 

 head which, being visible on the external surface of the arm, 

 contribute to the external form. 



The long portion, which is triangular in shape and of 

 considerable development, arises in the cat and the dog from 

 the inferior half or two-thirds of the posterior border of the 

 scapula (axillary border) ; from the whole extent of that 

 border as far as the superior posterior angle in the pig, the 

 ox, and the horse ; it then passes downwards towards the 

 articulation of the elbow, to terminate in a tendon which is 

 inserted into the olecranon process. The portion of this 

 muscle which is next the scapula is covered by the deltoid. 



The external head, situated below the long portion, is 

 directed obliquely downwards and backwards. It arises 

 from the curved crest which, from the deltoid impression 

 of the humerus, is directed upward to meet the articular head 

 of the same bone. This crest limiting the musculo-spiral 

 groove superiorly, and the brachialis anticus arising from the 

 whole extent of this groove, the result is that at this level 

 the external head is in relation with the brachialis anticus. 

 From this origin it is directed towards the elbow, to be 

 inserted into the olecranon, either directly or by the medium 

 of the tendon of the long portion. The part of this muscle 

 which arises from the humerus is covered by the deltoid. 



As for the internal head (Fig. 76, 4), which, in the super- 

 ficial layer, is only visible in its inferior part, on the internal 

 aspect of the arm in those animals in which the elbow is 

 free of the lateral wall of the thorax (the dog and the cat, 

 for example), it arises from the internal surface of the 

 humerus, and thence proceeds to be inserted into the ole- 

 cranon. 



* Other names given by certain authors to the parts of this muscle 

 which we have just enumerated still further complicate this nomenclature. 



The long head is further designated by them under the names of the 

 long or great anconeus ; the external head under those of external anconeus, 

 or lateral or short anconeus ; whilst the internal head becomes the internal 

 anconeus, or median. 



